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Abstract

The objective of this project is to reconstruct a picture of initial Laurentide Ice Sheet retreat at the end of the Last Glacial Maximum (LGM) using geochemical proxies in Gulf of Mexico sediments, and place the reconstruction into global perspective. The project asks two questions. (1) Can a time frame be established for initial retreat of the Laurentide Ice Sheet? (2) If so, how does the timing compare to that of other large ice sheets and mountain glaciers in both hemispheres?

Sediment core MD02-2550 from the anoxic Orca Basin offers excellent preservation and a high sediment accumulation rate. Twelve accelerator mass spectrometry 14C dates provide very good age control from 18.36 - 23.88 ka, the transitional period from glacial to deglacial conditions. Paired Mg/Ca and d18O from the planktonic foraminifera Globigerinoides ruber (pink variety) were combined with a matching record from the upper half of the same core from a previous study (Williams et al., 2010), expanding the record to 10.73 - 23.86 ka.

Sea surface temperature (SST) derived from Mg/Ca exhibits a mean value of 23.0 ± 0.8°C through the LGM (18.4-23.9 ka), ~3.9°C below the modern summer mean. At 18.4 ka, mean values drop in an anomalous cold snap, exhibiting a mean of 21.7°C that lasted until 17.8 ka. At 17.8 ka, SST begins a recovery warming toward present day conditions. This warming occurs markedly early relative to the onset of the Bølling-Allerød warm period, known best from Greenland ice core records.

The d18O of seawater exhibits no sustained shift toward more depleted values that would be consistent with a single major surge of initial meltwater. Instead, d18Osw appears to have been over 1.5 per mil below the modern mean throughout the LGM, persisting through the early deglacial period, and not shifting toward more positive values until well into the Younger Dryas. The corresponding salinity estimates were likewise ~2 psu lower than modern surface waters. Several negative excursions (~1 per mil) during the LGM and deglaciation coincide with millennial-scale retreats of individual lobes along the southern margin of the Laurentide Ice Sheet. These retreats and re-advances have previously been suggested to mirror small short-term excursions in Greenland ice core d18O, that reflects air temperature changes.

The consistently depleted d18Osw-ivc values and corresponding salinity estimates through the LGM require a mechanism to create a steady-state lower salinity environment in the northern Gulf of Mexico during the LGM, which would persist as SST changed.